METABOLISM OF GREEN PLANTS 37 



or, after it is done, how the protein becomes an intrinsic part 

 of the living material itself. So we attribute it to synthesiz- 

 ing ENZYMES. These are chemical bodies which are only 

 known as products of living protoplasm and are the activat- 

 ing agents (catalytic agents) for chemical transformations 

 in which, however, they themselves take no integral part. 

 The chemical composition and constitution of enzymes is 

 undetermined. 



Sphaerella thus takes the raw elements, so to speak, of 

 living matter and by the radiant energy of sunlight, which 

 its chlorophyll traps, constructs carbohydrate, protein, pro- 

 toplasm. In other words, the green plant is a synthesizing 

 agent, building up highly complex and unstable molecular 

 aggregates brimming over with the energy received from 



the Sun. 



2. Respiration 



So the green plant, whether Sphaerella or Elm, manu- 

 factures its own food and itself! But, as we have said before, 

 protoplasm is always at work to live is to work and 

 this means expenditure of energy, the same energy which 

 chlorophyll has secured for the plant and stored away in its 

 food. In other words, the food must be oxidized in order to 

 release the energy, and for this the plant must have available 

 a supply of free oxygen. Sphaerella obtains this oxygen dis- 

 solved in the water and, incidentally, in sunlight, from that 

 liberated through photosynthesis. The process involved, 

 for the sake of simplicity, may be represented by the equa- 

 tion: 



C 6 H 12 6 + 6 O 2 = 6 CO 2 + 6 H 2 O 



which, it will be noted, is the reverse of the equation for 

 photosynthesis. This intake of free oxygen by the cell and 

 outgo of carbon dioxide and water, the chief products of 

 combustion, is known as RESPIRATION. It is an interchange 



